The present invention relates to motors and, more particularly, to a forcer configuration for a linear motor and to a corresponding three phase linear motor system.
There are various configurations of linear motors, including generally flat motors, U-channel and tubular shaped motors. Different types of linear motors also are available, including brush, AC brushless, stepper, and induction motors. Common to most linear motors are a moving assembly, usually called a forcer, which moves relative to a stationary platen according to magnetic fields generated by application of current through one or more associated windings. The windings can be on the forcer or at the platen depending on the type of motor.
For example, in a permanent magnet linear motor, a series of armature windings are mounted within a stage that is movable relative to a stationary base plate or platen. The platen typically includes an array of permanent magnets configured to interact with the coils in the stage when energized with an excitation current. Alternatively, the magnets can be located in the stage with the coils situated in the platen. A closed loop servo positioning system is employed to control current through the windings. For example, current is commutated through coils of the stage with a three phase sinusoidal or trapezoidal signal in a closed loop feedback system. When such a linear motor is used in a positioning system, the relationship between the location of the stage and locations of the coils is utilized to control its operation. In such a linear motor, the available magnetic field intensity and thus the force is limited by the field strength of available motor magnets.
A linear stepper motor includes a forcer having windings that are inserted into a laminated core assembly. The stepper also includes a stationary platen having a plurality of teeth spaced apart from each other in a direction of movement. The forcer moves by application of power to a winding, which generates force by causing teeth of the forcer to align with teeth of the platen. The change in current through the windings causes the teeth to consecutively align and, thus, create linear motion. Because the forcer moves a predetermined amount based on the number current pulses, a stepper motor can function as an open loop system that does not require servo tuning. The number of pulses to create motion, which varies based on the tooth pitch on the platen, determines the resolution of the movement. In order to provide desired resolution and stiffness in a linear stepper motor, for example, platen tooth pitch of about 1 mm or less is required, which typically is formed by photochemical etching.
Linear motors are increasingly being employed in manufacturing equipment. In such equipment, nominal increases in the speed of operation translate into significant savings in the cost of production. However, the cost of such equipment often plays a decisive role in determining which type of system will be employed.
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended to neither identify key or critical elements of the invention nor delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.
One aspect of the present invention provides a forcer for a three phase linear motor system. The forcer includes three pole pairs, each having at least two poles that are magnetically coupled together as part of an associated phase of the motor system. Each phase includes a coil operatively associated with one or more of the poles, which coil can be energized to provide an electromagnetic field at the respective pole pair. Each pole also includes a set of teeth, where each set of teeth for the respective pole pairs are offset from each other to facilitate operation of the three phase linear motor system.
In accordance with a particular aspect, a first set of teeth for one pole pair is offset +/xe2x88x92120 degrees plus M*180 degrees and +/xe2x88x92240 degrees plus N*180 degrees, respectively, of a tooth pitch relative to the other sets of teeth for the other two pole pairs, where M and N are natural numbers.
Another aspect of the present invention provides a three phase linear motor system. The motor system includes a forcer that is moveable relative to a platen. The forcer includes three pole pairs, each pole pair having windings to define an associated phase of the motor system. The pole pairs are arranged to oppose associated teeth of the platen, which platen teeth provide a return path for magnetic flux from the pole pairs. The platen teeth are spaced apart from each other in a direction of movement according to a predefined tooth pitch. Each of the pole pairs also includes a set of teeth, with the set of teeth of the second pole pair being offset from the set teeth of the first pole pair by about +/xe2x88x92120 degrees of the tooth pitch. The set of teeth of the third pole pair are offset from the set of teeth of the first pole pair by a multiple of about xe2x88x92/xe2x88x92240 degrees of the tooth pitch.
To the accomplishment of the foregoing and related ends, certain illustrative aspects of the invention arc described herein in connection with the following description and the annexed drawings. These aspects are indicative, however, of but a few of the various ways in which the principles of the invention may be employed and the present invention is intended to include all such aspects and their equivalents. Other advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.